The therapeutic efficacy of TNF antagonists and anti-CD20 B cell depletion therapy in rheumatoid arthritis (RA) have altered pathophysiologic paradigms and generated new questions about underlying disease mechanisms. The observation that both TNF and B cells are of central importance indicates a direct relationship between these factors, particularly in regards to disease flares and inadequate treatment response. To address this, we have utilized the TNF-transgenic (TNF-Tg) mouse model of RA to developed: contrast enhanced (CE) MRI, near-infrared indocyanine green (NIR-ICG)-imaging and in vivo micro-CT in order to visualize and quantify changes in soft tissues, lymphatic flow, and bone during inflammatory- erosive knee arthritis. These results demonstrated that dramatic changes unfold in a predictable temporal sequence in the draining popliteal lymph-node (PLN), which appears to precede the onset of acute joint inflammation. Prior to detectable synvovial hyperplasia, the draining PLNs expand, which is followed by a sudden collapse of the node with translocation of B-cells to the paracortical sinusoids. This collapse phase, which is remarkably similar to LN shutdown observed during an immune response, is mediated by the CXCL13-CXCR5 chemokine-chemokine receptor axis. An unexpected finding was that LN collapse appears to preceded the onset of knee synovitis and focal erosions, and that preliminary studies showed that both anti-CD20 and anti-CXCL13 treatment of TNF-Tg mice resulted in increase PLN size and CE, and ameliorated arthritis. These observations could also be explained by synovial/subchondral lymphoid neogenesis, which triggers PLN shutdown and flare. Thus, we hypothesize that a pivotal event in the collapse of the PLN is the migration of B cells in response to CXCL13 expressed by monocytes/macrophages in the joint and/or in the PLN. Here in we propose to formally establish the cause- effect relationship between collapsed PLN and the induction of inflammatory-erosive arthritis. We will understand their relevance to human disease by: 1) establishing an association between decreased lymphatic flow with PLN collapse and/or lymphoid neogenesis in the joint; 2) elucidation of the specific role of B-cells and CXCL13 in PLN collapse and the induction of inflammatory-erosive arthritis using both loss- (anti-CD20 and anti-CXCL-13 therapy; TNF-Tg x mMT-/- and TNF-Tg x RAG1-/- mice) and gain-of-function (immunization and adoptive transfer, artificial induction of PLN collapse) experiments; and 3) examination of the translational relevance of altered lymph node dynamics in the murine model to human disease by analysis of PLN in the knees of RA patients with MRI and ultrasound before and after anti-TNF and B cell depletion therapy. PUBLIC HEALTH RELEVANCE: Rheumatoid arthritis (RA) is the most prevalent autoimmune disease, which affects ~1% of our population, and is largely manifested by episodic flares of joint inflammation, pain and tissue destruction. Based on our new discoveries in mice, and the clinical success of B-cell depletion therapy in RA patients who have inadequate responses to anti-TNF therapy, we propose that arthritic flare is caused by an immune reaction in adjacent lymph nodes that compromises joint drainage. Here we propose to test this theory in mouse models and RA patients.